1. Field of the Invention
This invention relates to the growth of large size or diameter, high quality, semiconductor-grade, single crystals of silicon carbide or other materials for optical and electronic device applications.
2. Description of the Prior Art
Single crystals of materials such as silicon carbide (SiC), at least in large diameters, are often grown in a physical vapor transport (PVT) reactor. A source of a growth material, such as a source powder of silicon, is placed at the bottom of a reaction chamber and a seed crystal is attached to the opposite surface of the reaction chamber, and above the source powder, to provide a base for further growth. The source powder is heated up in various known ways to produce a vapor, and a temperature differential is maintained between the source powder and the seed crystal. The vaporized powder condenses on the seed crystal and begins to form a solid, continuous mass of a single crystal of material of a particular polytype. Prior art patents in this area include, for example, U.S. Pat. Nos. 5,683,507; 5,611,955; 5,667,587; 5,746,827; and Re. 34,861.
It is desirable in many electronic and optical applications to provide a large size (over two inches) single crystal of SiC or other materials. The common technique used today for manufacturing these large size materials is to produce a large seed for material growth by a slow, gradual enlargement of the diameter of a small seed or starting crystal using a tapered sleeve in a graphite reactor hot zone. This known approach is expensive and time consuming since many growth cycles may be required to achieve relatively small increases in size, while maintaining good crystal quality. It is believed that this results from the outer crystal regions accumulating defects, such as inclusions, secondary grains and micro-pipes.
It is an object of the present invention to overcome the problems with the prior art and readily provide a large seed crystal for growing single crystals of SiC or other materials in a PVT reactor or the like.
We have invented a method for reproducibly reproducing large size, single crystals in a crystal growth chamber which includes the steps of forming a plurality of smaller size tiles of single crystals of substantially the same crystal orientation as the desired large size, single crystals, assembling the plurality of smaller tiles into a structure having a larger size while minimizing the gapping between adjacent tiles, placing the formed assembly of smaller tiles into a growth chamber and, through a growth reaction carried out in the growth chamber, forming a large size, single crystal using the assembly of smaller tiles as a seed crystal for the growth reaction.
The smaller tiles assembled together to form the seed crystal can either be assembled along substantially the same crystallographic orientation or can be assembled with alternating polarity between adjacent tiles. It is preferred that the smaller tiles be assembled into a structure and fixed to a holder which, in turn, is then placed in the growth chamber. The smaller tiles can be affixed to the holder with an adhesive, such as a sugar solution.
In a preferred embodiment, the growth reaction is first carried out to overgrow the surface of the assembly of smaller tiles and any gaps between adjacent tiles, and then the growth reaction is carried out to grow a large size crystal with the overgrown assembly of smaller tiles functioning as a seed crystal for the growth reaction.
The process of the present invention is particularly suitable for growing large size, single crystal silicon carbide from smaller size tiles-of single crystal silicon carbide. The growth reaction carried out in the growth chamber is preferably a physical vapor transport or sublimation reaction. In such an arrangement, the assembly of smaller silicon carbide tiles and a silicon crystal growth source are introduced into a sublimation system, and then the pressure of the system, the temperature of the assembly of smaller tiles, and the temperature of the crystal growth source are adjusted to cause the crystal growth source to vaporize and condense on the assembly of smaller tiles under the conditions required to form a single crystal.
The smaller tiles can be assembled together to form the seed crystal by first preparing a high quality surface on the smaller tiles so as to promote subsequent crystal growth, cutting the tiles to form polygonal tile pieces, assembling the polygonal tile pieces into an assembly of smaller tiles, affixing the assembled tiles onto a seed holder, and then placing the seed holder with the tiles affixed thereto into the growth chamber for subsequent large size, single crystal growth. It is generally preferred that the assembly of smaller tiles be shaped along its outer edges to form a generally circular structure so as to form a generally circular, large size, single crystal.
In one embodiment for growing silicon carbide single crystals, the smaller tile pieces are assembled so that either all of their C-faces or all of their Si-faces are oriented toward the vapor in the growth chamber during the growth reaction process. Alternatively, only some of the C-faces and some of the Si-faces are so oriented, with the ratio of C- and Si-faces so oriented chosen so as to improve growth in the reaction process.
A complete understanding of the invention will be obtained from the following description when taken in connection with the accompanying drawing figures wherein like reference characters identify like parts throughout.